1. Field of the Invention
The present invention relates to a novel catalytic method for preparation of thiophenols from phenols and hydrogen sulfide at low, e.g. atmospheric pressure.
2. Prior Art
At least as far back as 1910 catalytic methods have been described for making thiols from alcohols and hydrogen sulfide, e.g. in Comptes Rendus, 150 (1910), pp. 1217-1221, Paul Sabatier and A. Mailhe, "General Method of Direct Preparation of Thiols By Catalysis, Starting From Alcohols." The catalyst was thoria and good thiol yields were obtained from primary alcohols, but the yields from secondary alcohols were about one third those of the primary alcohols. By passing a mixture of phenol or a cresol and hydrogen sulfide over thoria heated to between 430.degree. and 480.degree. C. the corresponding thiophenol was obtained; however, the yield obtained was less than that obtained with the alcohols and in the best case was only 17%. Above 500.degree. C. the yield decreases due to destruction of H.sub.2 S.
In that same year 1910 another article was published by the same authors, Comptes Rendus, 150 (1910), pp. 1569-72, "On the Formation and Condensation of Thiols: Synthesis of Thioethers." A translation of a portion of this article reads as follows:
"In a recent communication we have shown a general method for the direct preparation of thiol, by the reaction of hydrogen sulfide and alcohol vapors in contact with thorium oxide above 300.degree. C. This procedure, very advantageous for primary alcohols, gives poorer yields with secondary alcohols, and its results are less satisfying for phenols for which a very high temperature is required and only very mediocre yields are obtained."
"There was a certain interest to learn if the other anhydrous metallic oxides which our previous research had shown as capable of causing catalysis of alcohols, would be more or less preferable to thoria for the direct synthesis of thiols. From this point of view, we have examined two oxides which are exclusively dehydration catalysts, alumina and the blue oxide of tungsten, and various oxides which are mixed dehydration and dehydrogenation catalysts, chromium oxide, zirconium oxide, uranium oxide and the blue oxide of molybdenum, all used in their most active form."
"For phenol itself, the temperature of the oxide being in the vicinity of 450.degree. C., the thiophenol yield was lower in every case. The yields found were:
Aluminum oxide -- 0.4%
Zirconium oxide -- 1.5%
Blue molybdenum oxide -- 1.8%
Blue tungsten oxide -- 1.5%
Chromium oxide -- 2.5%
Uranium oxide -- 3.8%
Thoria gave a yield of 8% and therefore was confirmed as being very superior to the other oxides, the same as for alcohols."
In U.S. Pat. No. 2,116,182, Baur, patented May 3, 1938, "Production of Mercaptans", production of aliphatic mercaptans of high molecular weight is described and carried out by reacting an alcohol and hydrogen sulfide in the presence of a dehydration catalyst. The catalysts which are shown in the experimental examples are zirconium dioxide which it is stated may be supported on pumice, activated charcoal which can be impregnated with phosphoric acid to improve activity, aluminum oxide and granulated titanium dioxide. In the patent, column 1, lines 36 to column 2, line 5, the catalysts are discussed more broadly as to what other catalysts may be useful.
In U.S. Pat. No. 2,438,838, Ballard et al., patented Mar. 30, 1948, "Process for the Production of Thiophenols" a process is described involving reacting a phenol with hydrogen sulfide at superatmospheric pressure in the presence of a metal oxide dehydration catalyst.
Superatmospheric pressure is described in column 1, line 41 to column 2, line 10, which are generally described as above 200 psi as being sufficient, and in most cases pressures above about 300 psi are preferred. The patent goes on to say that pressures above approximately 400 psi have been found effective in producing substantial yields at temperatures between 400.degree. C. and about 600.degree. C. Catalysts which may be used are broadly discussed beginning in column 2, line 46 and continuing through column 4, line 22. In the experimental examples only two catalysts were used; namely, activated bauxite and activated alumina.
None of the above prior art experimentally tried the particular catalyst, vanadium oxide, which is the catalyst used in the process of this invention, and in only one of the reference is vanadium oxide mentioned in the broad teachings, namely U.S. Pat. No. 2,116,182, column 1, line 43 involving a process for the production of mercaptans, not thiophenols. Clearly the process of this invention is both novel and unobvious over the teachings of the prior art.